Characterization of the known Mycoplasmas by biochemical and physiological properties, DNA base composition, and nucleic acid homology will be continued to determine the extent of the fundamental heterogeneity that cleaves the group, and to delineate subgroups of similar mycoplasmas. A relationship uncovered between mycoplasmas and streptococci will be examined to determine whether similarities between these organisms are coincidental or due to a true evolutionary relationship. Another study area will be examination of mycoplasma envelopes and the effect of envelope composition on host response to mycoplasmas. Bibliographic references: Neimark, H. 1974. Biochemical and Immunological Relationships Between Sterol-nonrequiring Mycoplasmas and Streptococci. Abstract of the Ann. Meeting, Amer. Soc. for Microbiology, p. 108; Neimark, H. 1974. Implications of the Phylogenetic Relationship between Acholeplasmas and Lactic Acid Bacteria. In Mycoplasmas of man, animals, plants and insects. An INSERM International Congress, eds. Bove, J.M. and Duplan, J.F. (INSERM, Paris), Vol. 33, pp. 71-78.